1. Field of the Invention
The present invention relates to a method for the purification of alkyl nitrite-containing exhaust gases by treatment with amidosulphuric acid.
Alkyl nitrites (alkyl esters of nitrous acid), in particular the readily volatile lower alkyl nitrites, are used as reagents for oximations, nitrosations and diazotizations (Houben-Weyl Vol. 6/2 pp. 334-362).
Furthermore, a number of industrially interesting continuous methods are known in which the formation of alkyl nitrites, in particular methyl nitrite, proceeds in the gas phase within an overall process which is characterized in that alkyl nitrites, in particular methyl nitrite, function in the overall process as oxidants, cocatalysts, alkoxylation reagents or other reaction partners. It is typical of such processes that in their course the nitrogen monoxide formally contained in the alkyl nitrite is not consumed but is released as a gas. In a preferred embodiment of such reactions, the alkyl nitrite just generated is introduced into the reaction chamber or the reaction vessel together with the additional reaction partner or additional reaction partners and auxiliaries and the portion of the product gas stream which remains in the gaseous state and contains the nitrogen monoxide formed in the course of the reaction is returned to the reactor for the preparation of alkyl nitrite after separating off to the greatest possible extent the condensed or condensable reaction product or products. A cyclic process with respect to nitrogen monoxide and the particular alkyl nitrite is thus closed. The principle of such a process as exemplified by the use of methyl nitrite is depicted in FIG. 1. In detail, FIG. 1 shows a reactor I for the preparation of alkyl nitrite and a reactor II for a reaction consuming alkyl nitrite. The mass streams are 1=alkyl nitrite just formed, 2=other reaction partners for the reaction in II, 3=the reaction product formed in II and isolated by condensation and 4=the exhaust gas formed in the reaction in II and containing NO for return to I.
2. Description of the Related Art
Reactions in which such a continuously operated cyclic procedure is particularly advantageous are, for example, the following:
(A) The preparation of dimethyl oxalate from carbon monoxide and methyl nitrite in the presence of suitable catalysts (cf. EP 46 598) ##STR1##
(B) The oxidation of unsubstituted or substituted olefins in the presence of methanol and suitable catalysts (cf. EP 55 108) ##STR2##
(C) The preparation of dimethyl carbonate from carbon monoxide and methyl nitrite in the presence of suitable catalysts (cf. EP 425 197) ##STR3##
If only an incomplete conversion is achieved when a low-boiling alkyl nitrite is used in the context of diazotizations, nitrosations or oximations, alkyl nitrite-containing exhaust gases occur.
Likewise, when alkyl nitrites are used as regenerable oxidizing agents, in particular in the case of the use of methyl nitrite mentioned by way of example in (A), (B) and (C) and reproduced diagrammatically in FIG. 1, it may be necessary to take off relatively small portions from the gas circulation to avoid enrichment of by-products.
The occurrence of uncontrolled reaction conditions in handling alkyl nitrites is also associated with the risk of the production of relatively large amounts of alkyl nitrite-containing exhaust gases.
For the industrial handling of alkyl nitrites in the context of the above described reactions it is therefore necessary, owing to the toxicity of the alkyl nitrites, to create apparatuses for the safe disposal of these alkyl nitrite-containing exhaust gases.
In JP 59/163 349 (CA 102 95248y), a method is described in which methyl nitrite is disposed of by introduction into an acidic hydrogen peroxide solution. This method is not industrially useable owing to the possible formation of explosive oxygen/methyl nitrite mixtures or other readily decomposable compounds.
To remove excess inorganic nitrites or nitrous acid from liquid reaction batches, the use of amidosulphuric acid has been proposed. Thus, for example, in DE 35 06 825, the purification of nitrite-containing wastewater by injection beneath the surface of excess aqueous amidosulphuric acid is described. As a result, the frequently observed formation of nitrous gases is to be suppressed.
However, there are no statements as to whether this method can be applied to the much more difficult--since it proceeds in heterogeneous reaction--removal of alkyl nitrites from gas streams and therefore there are also no statements on the industrial realizability of this possible method.